This invention relates to the treatment of industrial fly ash and more particularly to the treatment of fly ash to recover valuable trace metals such as gallium, silver, and the like.
In many industrial processes, fly ash is produced in the chimneys or stacks where electrostatic precipitators or other recovery equipment is used to remove particulates. In general, the fly ash is composed of fine grained particles having a silicate base with small amounts of trace metals on the surfaces of the particles. Two fly ashes produced in quantity are phosphate fly ash associated with the production of elemental phosphorous and coal fly ash associated with the combustion of coal.
For phosphate fly ash, the following reaction may illustrate the formation of a silicate base EQU 4Ca.sub.3 F(PO.sub.4).sub.3 +18SiO.sub.2 +30C .fwdarw.18CaSiO.sub.3. 1/9CaF.sub.2 +30CO.uparw.+3P.sub.4 .uparw.
Frequently fly ash contains metals such as gallium, silver, cadmium, manganese, lead, titanium and the like in trace amounts totalling less than about 1 wt. %. Recovery of one or more of these metals is important since the metals are either in limited supply within the United States, are expensive or may be objectional in the fly ash. With respect to gallium which is a strategic metal, it has been estimated that about 60% of gallium is imported. Other metals such as silver are relatively expensive and have many industrial uses. Still other metals such as cadmium may be objectional in the fly ash if it is used for ground fill or other environmental purposes.
Processes have been developed for recovery of some of these metals. As disclosed in German Offen. 2,407,948 associated with U.S. application Ser. No. 333,928, gallium is concentrated by contacting the fly ash with Ca(OH).sub.2 and removing the precipitate to provide a gallium containing solution. Carbon dioxide or hydrogen sulfide is then used to form a gallium containing precipitate. In Belgium 853,608 associated with Canadian application No. 250,752, the fly ash is heated with concentrated H.sub.2 SO.sub.4 to provide a solution containing gallium and zinc. The solution is treated with (NH.sub.4).sub.2 SO.sub.4 to precipitate ZnSO.sub.4 which is removed by filtration with the filtrate being treated with NaOH to precipitate gallium. While these processes may have value, they include several treatment and recovery stages and therefore have some complexities and expense.
Accordingly, one object of this invention is a process for the recovery of one or more trace metals from industrial fly ash. Another object of the invention is the recovery of gallium from industrial fly ash and particularly phosphate fly ash. An additional object of the invention is the removal of one or more trace metals from fly ash to provide a more acceptable product with respect to environmental uses. These and other objects of the invention will become apparent from the following detailed description.